This invention relates to a temperature control device for controlling the temperature of a thermal head used in a thermal printer, and more particularly to controlling the temperature of the heating elements in a thermal head by varying the electric current supplied to the head as a function of the printing conditions.
It is known that the temperature of the heating elements of a thermal printing head at the start of printing differs from their temperature during the course of printing. This will occur even though the amount and duration of current in the heating elements is the same for both instances. This happens because the temperature of the heating element drops after the start of the printing operation. Further, the optimum temperature for printing may vary depending upon the form of the characters that are being printed, such as a bar code, ordinary characters or the like.
A number of techniques have been employed to achieve optimum printing under varied conditions. One approach was to eliminate the difference between the temperature at the start of printing and that during the course of printing. This is done by preheating the thermal head at the time that the print head power supply is turned on, thereby maintaining the start temperature at approximately the same level as the print temperature. However, while this technique will control the temperature during the course of printing, it does not control the temperature in accordance with the nature of the characters being printed by the heating elements.
In order the heating elements to be at a temperature that is suitable for the characters being printed, there are known techniques for appropriately varying electric current as a function of printing ordinary characters or bar codes. Such techniques employ power supply circuitry which can generate two or three different voltages. The flow of current through the heating elements is changed by switching circuits to vary the voltage according to the nature of what is being printed. While this permits some measure of heating temperature control, fine control of temperature during printing is still not possible, because switching is still limited to two or three different voltages. An example of the problem is illustrated by the Bar Code I shown in FIG. 1. As the printing proceeds in the direction of the arrow, the temperature of each heating element rises as the printing progresses. This causes gradual thickening toward the end of the bar.
Another technique of temperature control is to detect the temperature of the thermal head and adjust the power supply circuit, so that the power supplied to the thermal head is in an amount which is appropriate to the required temperature. This allows a more precise level of control, as the right amount of power in view of the current temperature, is supplied. However, this technique is flawed since it is not detecting the actual change in temperature of each heating element. What is instead detected is the heat of the head board for the heating elements. It is therefore difficult to detect temperature changes precisely. Additionally, rapid fine temperature control is impossible because of thermal delay caused by the time it takes the heat to be conducted by the head board and the like. This technique therefore does not present an easy solution to the bar thickening problem seen in FIG. 1.